.box1 {
    width: 100px;
    height: 100px;
    background-image: url(../images/1.jpeg);
}

.box2 {
    width: 100px;
    height: 100px;
    background-image: url(../images/2.png);
}

.box3 {
    width: 100px;
    height: 100px;
    background-image: url(../images/3.gif);
}